3 2. BIOLOGY AND ECOLOGY OF SMALL TUNAS IN THE MEDITERRANEAN AND THE BLACK SEAS This section of the report includes the available information on the life history of the main species of small tunas present in the Mediterranean Sea and in the Black Sea, according to various scientific sources, with particular attention to the biological parameters useful for stock assessment. As far as the biology is concerned, it was decided only to take into account the specific features reported for the study area, substituting them with worldwide references if the local information was not available. Length frequencies have been collected by several fisheries and they have been summarized herein by species. 2.1 Sarda sarda (Block, 1793) The Atlantic bonito, Sarda sarda (Block, 1793) (ICCAT code BON) is an epi-pelagic neritic schooling species which lives in a large range of water temperatures (12–27°) and salinities (14–39‰), sometimes reported entering estuaries (Collette and Nauen, 1983). Its distribution is in tropical and temperate waters of the Atlantic and Mediterranean, including the Black Sea. On the East side of Atlantic the distribution appears uninterrupted from Scandinavia to South Africa; on the West side, it presents interruptions in the Caribbean Sea and South of the Amazon River to Northern Argentina. The maximum size in the Atlantic is 91.4 cm (Collette and Nauen, 1983), in the Mediterranean it is 96 cm (Ionian Sea, De Metrio et al., 1998) and in the Black Sea it is 90 cm (Kara, 1979). The diagnostic features are well known (Collette and Nauen, 1983): upper jaw teeth 16 to 26; lower jaw teeth 12 to 24; gillrakers 16 to 23 on first arch. Dorsal fin 20–23 spines; dorsal finlets usually 8; anal fin 14–17 rays; anal finlets usually 7; pectoral fin 23–26 rays. Vertebrae: 26–28 precaudal plus 23–27 caudal. The meristics of dorsal fin and vertebrae are higher than the other three species of Sarda [S. australis (Macleay, 1880), S. chiliensis (Cuvier, 1831), S. orientalis (Temminck and Schlegel, 1844). More detailed information about biometric and meristic characteristics can be found in Demir (1964) and Franicevic et al. (2005). 2.1.1 Migrations In the Western and Central Mediterranean Atlantic bonitos are mainly fished in coastal waters, but large specimens (60–85 cm FL) are also present offshore; observations made while studying the swordfish fishery in the Italian waters ascertained a distance from the coast of about 15 nm, at a depth of more than 2 000 m in the Ligurian Sea and a large distribution offshore, even over very deep bottoms in the Central and Southern Tyrrhenian Sea. 4 In the Eastern Mediterranean migrations from the Black Sea to the Aegean Sea and viceversa have been studied since the fifties by tagging techniques (Demir, 1957). The water temperatures possibly influencing fish movements were also recorded (Acara, 1957). There are large spawning grounds in the Black Sea, which give huge quantities of young fish not only moving along the Southern coast of the same sea, but also migrating in autumn to the Marmara Sea and in part to the North Aegean. Where the Black Sea is concerned Atlantic bonito moves to the southern Black Sea coast in May–July, forming shoals and staying in the same area from the autumn/winter period until the beginning of March. Individuals of age 1 probably migrate to the same region for feeding. All these locations represent fishery areas of the Turkish fleet (Oray, Karakulak and Zengin, 1997). During the spring, with a reverse migration, adult fish reach the spawning areas of the Black Sea; restrictions have been enforced on fishery from April to September to protect the spawning season. Each year class strongly influences the production and can trigger oscillations within a period of several years. In the seventies severe environmental decay occurred in the Black Sea and since then large migratory species such as bluefin, swordfish, little tunny and bluefish have disappeared. The Atlantic bonito is no longer available throughout the area, but is apparently limited mainly to the southern part of this sea. Atlantic bonitos tagged on the Spanish Mediterranean coast (Rey, Alot and Ramos, 1984) have shown that: 1) the fish can move along the coast in both South and North directions; 2) a specimen covered about 370 nm in less than 4 months, travelling towards Gibraltar from Castellon to Estepona (Rey and Cort, 1978). A consistent fraction of fish tagged at the tuna trap of Ceuta was recovered in the Atlantic, both South and North of the Straits, from Morocco to Portugal (Rey and Cort, 1981). According to these scientific data, the distribution of the local population of S. sarda would not seem to be strictly confined to the Mediterranean Sea, but it is so far not known if Atlantic specimens (which have their spawning grounds along the Atlantic coast of Morocco (Dardignac, 1962) also move across the Strait toward the Mediterranean. It is likely that, from a management point of view, the Mediterranean and Black Sea stock can be considered as separate management units from the Atlantic stock, even if the exchange rate between the Atlantic Ocean and the Mediterranean and between the Mediterranean and the Black Sea are not known. The boundaries of this stock along the eastern Atlantic coast outside Gibraltar are yet to be defined. According to the data presented in this report, sub-stocks might be distinguished in the Western-Central and Eastern side of the Mediterranean Sea including the Black Sea. 2.1.2 Biological characteristics Many studies on the fishery biology of Sarda sarda have been carried out in the Eastern area (Zusser, 1954; Nümann, 1955; Slastenenko, 1956; Demir, 1957, 1963, 1964; Krotov, 1957; Mayorova and Tkacheva, 1959; Porumb and Porumb, 1959; Nikolov, 1960; Demir and Demir, 1961; Kutaygil, 1965). Recent data on the biological characteristics of this species, with implications for its management, have been studied both in the Eastern and Western Mediterranean and allow for the comparison of potentially different population units. According to a recent summary of information about Atlantic bonito of the Western and Central Mediterranean, a possible stock unit on the basis of genetic difference (Viñas, Alvarado Bremer and Pla, 2004) might be present in these areas (Orsi Relini et al., 2005). The length/weight relationships were studied by several scientists and the published findings are shown in Table 2 and in Figure 1. It is known that more recent data have been collected by some EC countries thanks to the EC-DCR national programmes, but these results have not been published yet. 5 Table 2 – Length/weight relationships in Sarda sarda in the Mediterranean and Black Seas Range FL Range W Function N Author Area Notes (cm) (g) W=0.01486LF2.9719 165 40–55.5 Rodriguez-Roda, 1966 Gibraltar – Aegean, Marmara, Black W=0.02361LF2.8703 1608 14–90 80–7500 Kara, 1979 Sea Rey, A lot and Ramos, W=0.00724LF3.1644 878 19–72 200–5500 Gibraltar (Med-Atl) M+F+Indet. 1984 Rey, A lot and Ramos, W=0.00653LF3.1865 242 33–65.2 436–4040 Mediterranean Atlantic M 1984 Rey, A lot and Ramos, W=0.00844LF3.1218 229 33–70.5 460–4866 Mediterranean Atlantic F 1984 W=0.0252LF2.83 845 – – Giacchetta et al., 1995 Gulf of Taranto M+F Oray, Karakulak and W=0.0039LF3.3263 – 21.8–70.5 110–5000 Turkey – Zengin, 2004 W=0.0038LF3.3414 285 35–67 – Franicevic et al., 2005 Adriatic Sea M W=0.0056LF3.2364 353 33–64.5 – Franicevic et al., 2005 Adriatic Sea F W=0.0085LF3.1230 665 33–67 – Franicevic et al., 2005 Adriatic Sea M+F W=0.03LF2.8323 240 35–82 700–7050 Di Natale et al., 2006 Tyrrhenian Sea – W=0.4LF2.1813 109 35–67 800–4000 Di Natale et al., 2006 Strait of Sicily – Spanish Mediterranean W=0.0094632LF3.1011 – – – Macias et al., 2006 – Traps 16000 15000 Rodriguez-Roda, 1966 14000 13000 Kara, 1979 12000 Rey et al., 1984 11000 Giacchetta et al., 1995 10000 Oray et al., 2004 9000 Franicevic et al., 2005 8000 Di Natale et al., 2006 Weight (g) 7000 6000 Di Natale et al., 2006 5000 Macias et al., 2006 4000 3000 2000 1000 0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 FL (cm) Figure 1 – Length/weight relationships of Sarda sarda in the Mediterranean Sea The reproductive season shows a remarkable variability, according to several authors. It is likely that it occurs largely from May to July in most of the Mediterranean Sea, with some yearly variation in March or April according to the areas, concentration or oceanographic features. It the Black Sea reproduction takes place in the second part of the spring, sometimes extending up to July. The optimal water temperature for spawning in the Black Sea is 18 °C (Majorova and Tkecheva, 1960). 6 Table 3 – Spawning periods and grounds of Sarda sarda in the Mediterranean and Black Sea AREA PERIOD AUTHOR Sicily May 20 – June 30 Sanzo, 1932 Algerian coasts March – May Dieuzeide, 1955 Gibraltar May – July Rodriguez-Roda, 1966 Black Sea May to mid June Demir, 1957 Black Sea June to mid July Mayorova and Tchaceva, 1959 Mediterranean and Atlantic June – July Rey et al., 1984 Morocco Catalan coast May to July Sabates and Recasens, 2001 Southern Tyrrhenian Sea May to July Di Natale (pers.com.) Straits of Sicily April to June Di Natale (pers.com.) Ligurian Sea May to July Orsi Relini (pers.com.) When describing basic characteristics of S.